In X-ray imaging, for example in medical X-ray imaging, it is necessary to control radiation levels. Therefore, the X-ray dose is measured, for example by providing an ionization chamber. The ionization chamber may be provided as an extra-unit between the object, for example a patient, and the detector/x-ray source. However, due to the ionization chamber provided as a separate component, the setup for the X-ray imaging system consumes valuable space. Further, the ionization chamber may be displaced and exposure procedures may be performed without ionization chambers.
FIG. 1 is a schematic view of a simplified X-ray source housing 100 according to prior art. The X-ray source housing 100 comprises an X-ray tube 101, which generates X-rays 102. The X-rays 102 pass through a collimator 103 and in this case a Dose Area Product (DAP) meter 104.
DAP meters are usually large-area, transmission ionization chambers and associated electronics. In use, the ionization chamber is placed perpendicular to the beam central axis and in a location to completely intercept the entire area of the x-ray beam. The DAP, in combination with information on x-ray field size can be used to determine the average dose produced by the x-ray beam at any distance downstream in the x-ray beam from the location of the ionization chamber.
DAP is defined as the integral of dose across the X-ray beam. Therefore DAP includes field non-uniformity effects such as anode-heel-effect, and the use of semi-transparent beam-equalizing shutters (lung shutter). Assuming that the incident beam is totally confined to the patient, the recorded value may essentially provide an upper limit on the X-ray energy absorbed by the patient (i.e. there is no transmission or scatter). DAP's ability to estimate stochastic risk is degraded because of the lack of dose distribution information within the patient. The best may be to assume an average weighting factor for all the tissues at risk. This may lead to an over or under estimate of risk in certain cases.